1. Field of the Invention
The present invention relates to an image processing method and apparatus, and a program for realizing an image processing function on a computer, and more particularly to a favorable halftone image processing technique for an inkjet recording apparatus or another image forming apparatus.
2. Description of the Related Art
Typically, with an inkjet recording apparatus, grayscale images are formed by converting the shading of an image into an appropriate dot placement using a halftoning method typified by an error diffusion method and a dithering method (see Japanese Patent Application Publication Nos. 2001-157055, 2003-110852, 7-15606, etc.).
Japanese Patent Application Publication No. 2001-157055 proposes a method of evaluating the noticeability of dots quantitatively, quantizing the noticeable dots using an error diffusion method, and quantizing the other dots using a systematic dithering method. Japanese Patent Application Publication No. 2003-110852 proposes a halftone processing method in which a process of diffusing an error generated by quantization through a threshold matrix to peripheral unprocessed pixels is repeated successively. Japanese Patent Application Publication No. 7-15606 proposes a method in which a process of grouping gray value images into pixel (image element) line units and moving the quantization error of a subject pixel in a certain line to an adjacent pixel in the next line is repeated successively.
As described in Japanese Patent Application Publication Nos. 2003-110852 and 7-15606, a quantization process of diffusing an error generated by quantization through a threshold matrix to peripheral pixels is advantageous in that it enables high quality image reproduction, but is disadvantaged in that the calculation load required for successive quantization error diffusion is large. Furthermore, since sequential processing is required, parallel processing is difficult, and it is therefore difficult to increase the processing speed.